Reliability of operation is an important consideration for modern electronic devices, e.g., selective call receivers. One aspect of reliability is the device's ability to continue to function properly after sudden mechanical impacts and shocks, e.g., dropping the unit onto a hard surface. Modern selective call receivers, e.g., pagers, generally include relatively thin printed circuit boards, housings which are typically made of a plastic type material, and fragile electronic components. The plastic housing's front and back planes and internal printed circuit boards mounted within the housing typically have a low mechanical frequency response to sudden impacts, resulting in relatively large deflections. The deflecting front and back planes, as well as the deflecting printed circuit boards, can impact with each other, resulting in primary and secondary impacts with the components supported by the printed circuit boards. Certain ones of these components are fragile in nature, e.g., constructed of quartz, ceramic, and silicon, making them especially susceptible to failure due to mechanical shocks. Additionally, each of these components also has a natural mechanical frequency response to impact that can amplify the incoming shock and cause serious damage to the component.
Furthermore, modern low volumetric selective call receivers, e.g., such as in credit card form-factors, do not permit height tolerances between the printed circuit boards and the housing front and back planes to accommodate large deflections. As a result, sudden mechanical shocks typically cause primary and secondary impacts between the deflecting structures. This can result in unit failures. For example, large impacts, whether primary or secondary, can create detached or broken solder joints in integrated circuits, ceramic filters, and other components. Further, excessive printed circuit board deflections can overstress and fatigue solder joints resulting in failure.
The current method of providing shock isolation within a selective call receiver is to place one or more pieces of shock isolating material in selected areas. Unfortunately, this approach has provided a limited amount of shock isolation in a single direction only, and does not solve all of the problems described above. One additional problem with this approach is that variations in thickness of the housing front and back, as well as variations in thickness of the printed wiring board and of the shock isolating material itself, can produce tolerance build-ups that compress the shock isolating material enough to cause damaging force on housing attachment mechanisms. Further, if during manufacturing of the selective call receiver, one or more of the pieces of shock isolating material are not correctly placed or missing in the selected areas, the final delivered product is again susceptible to failures due to mechanical shock as discussed above.
Thus, what is needed is an apparatus for isolating the electronic device and its constituent parts from mechanical shock by reducing the deflections of the constituent parts. Furthermore, the apparatus should accommodate expected variations in component thickness without damage to housing attachment mechanisms. Preferably, the electronic device should also externally indicate if the shock isolating apparatus is internally misplaced or missing to reduce the possibility for manufacturing defects and to enhance the reliability of the delivered product.